feat(web): auto-toast on API errors with dismissible notifications

Add showToast() utility in api.js and a matching toast stylesheet.
apiFetch now automatically shows a toast for any 400+ response before
re-throwing, so callers can still .catch() for additional handling.

Toasts stack at the bottom-right, auto-dismiss after 4s, and support
error/warning/success/info levels via a left-border colour accent.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/js/api.js

@@ -4,6 +4,54 @@ export function setStatus(text) {
   dom.statusText.textContent = text;
 }
 
+// ── Toast ─────────────────────────────────────────
+
+function getContainer() {
+  let el = document.getElementById("toast-container");
+  if (!el) {
+    el = document.createElement("div");
+    el.id = "toast-container";
+    document.body.appendChild(el);
+  }
+  return el;
+}
+
+const ICONS = { error: "✕", warning: "!", success: "✓", info: "i" };
+
+/**
+ * 显示 toast 通知。
+ * @param {string} title       主要文字
+ * @param {object} [opts]
+ * @param {string} [opts.message]  次要说明文字
+ * @param {"error"|"warning"|"success"|"info"} [opts.level="error"]
+ * @param {number} [opts.duration=4000]  自动关闭毫秒数，0 表示不自动关闭
+ */
+export function showToast(title, { message, level = "error", duration = 4000 } = {}) {
+  const container = getContainer();
+
+  const el = document.createElement("div");
+  el.className = `toast ${level}`;
+  el.innerHTML = `
+    <span class="toast-icon">${ICONS[level] ?? "i"}</span>
+    <div class="toast-body">
+      <div class="toast-title">${title}</div>
+      ${message ? `<div class="toast-message">${message}</div>` : ""}
+    </div>`;
+
+  const dismiss = () => {
+    el.classList.add("hiding");
+    el.addEventListener("animationend", () => el.remove(), { once: true });
+  };
+
+  el.addEventListener("click", dismiss);
+  container.appendChild(el);
+
+  if (duration > 0) setTimeout(dismiss, duration);
+  return dismiss;
+}
+
+// ── apiFetch ──────────────────────────────────────
+
 export async function apiFetch(url, options = {}) {
   const response = await fetch(url, {
     headers: { "Content-Type": "application/json" },
@@ -11,7 +59,9 @@ export async function apiFetch(url, options = {}) {
   });
 
   if (!response.ok) {
-    throw new Error((await response.text()) || response.statusText);
+    const text = (await response.text()) || response.statusText;
+    showToast(`请求失败 ${response.status}`, { message: text });
+    throw new Error(text);
   }
 
   if (response.status === 204) {

web/styles.css

@@ -1000,6 +1000,63 @@ button.danger:hover { background: var(--danger-hover); }
   color: var(--text);
 }
 
+/* ── Toast ────────────────────────────────────────── */
+
+#toast-container {
+  position: fixed;
+  bottom: 20px;
+  right: 20px;
+  display: flex;
+  flex-direction: column-reverse;
+  gap: 8px;
+  z-index: 9999;
+  pointer-events: none;
+}
+
+.toast {
+  display: flex;
+  align-items: flex-start;
+  gap: 8px;
+  min-width: 240px;
+  max-width: 380px;
+  padding: 10px 14px;
+  background: var(--surface);
+  border: 1px solid var(--border);
+  border-left: 3px solid var(--text-3);
+  box-shadow: 0 4px 12px rgba(0,0,0,0.1);
+  font-size: 13px;
+  color: var(--text);
+  pointer-events: auto;
+  animation: toast-in 0.15s ease;
+  cursor: pointer;
+}
+
+.toast.error   { border-left-color: var(--danger); }
+.toast.warning { border-left-color: var(--warning); }
+.toast.success { border-left-color: var(--success); }
+
+.toast-icon {
+  flex-shrink: 0;
+  font-size: 14px;
+  line-height: 1.5;
+}
+
+.toast-body { flex: 1; word-break: break-word; }
+.toast-title { font-weight: 600; margin-bottom: 2px; }
+.toast-title:only-child { margin-bottom: 0; }
+.toast-message { color: var(--text-2); font-size: 12px; }
+
+.toast.hiding { animation: toast-out 0.15s ease forwards; }
+
+@keyframes toast-in {
+  from { opacity: 0; transform: translateY(8px); }
+  to   { opacity: 1; transform: translateY(0); }
+}
+@keyframes toast-out {
+  from { opacity: 1; transform: translateY(0); }
+  to   { opacity: 0; transform: translateY(8px); }
+}
+
 /* ── Scrollbar ────────────────────────────────────── */
 
 ::-webkit-scrollbar { width: 5px; height: 5px; }